Atherosclerosis leading to coronary vascular disease is a primary cause of mortality in the developed world. Atherosclerotic risk has been shown to be directly related to elevated plasma cholesterol levels. In plasma about 70% of cholesterol is esterified to long-chain fatty acids to form cholesteryl esters and these cholesteryl esters are bound to plasma lipoproteins. The lipoproteins involved in the transport of cholesterol and cholesteryl esters include low density lipoprotein (LDL), high density lipoprotein (HDL), and very-low density lipoprotein (VLDL).
While high levels of cholesterol associated with LDL have been linked to atherosclerotic risk (Schaefer et al., 1995), high HDL cholesterol levels may be protective against the development of heart disease (Miller et al., 1977). As a result there has been significant effort to develop therapies which effectively reduce the level of LDL cholesterol and raise the level of HDL cholesterol within an animal. HDL may play an anti-atherogenic role by promoting the clearance of cholesterol from the body (Eisenberg, 1984). Furthermore, Schwartz et al. (1978) disclose that cholesterol in HDL is specifically targeted for excretion from the body by the liver in the form of bile. However, current therapies directed to reduce the level of LDL cholesterol and raise the level of HDL cholesterol have not met with success.
The factors that regulate cholesterol flux to the liver are poorly understood but may involve two distinct systems; a cellular sterol regulatory system and an intravascular transport system. Excess extrahepatic cholesterol may be transported in HDL particles to the liver for excretion (Glomset, 1968). HDL has also been shown to be able to adsorb cholesterol and cholesteryl esters (CE) from cell membranes (Phillips et al., 1998). In addition, a second sterol transport pathway may include transfer of cholesterol from HDL to the rapidly turning over VLDL lipoprotein pool, followed by clearance of cholesterol by the liver (Tall, 1998).
The mechanism of intravascular sterol transport is also poorly understood, but may involve the concerted action of multiple proteins and enzymes. Two enzymes thought be involved in intravascular sterol transport are lecithin:cholesterol acyltransferase (LCAT) and cholesterol ester transfer protein (CETP). At present it is thought that LCAT may form a concentration gradient to move sterol into and through the blood plasma compartment by promoting the conversion of free cholesterol (FC) to cholesterol esters (CE) on HDL particles (Jonas, 1987). CETP may then promote this lipid flux by moving the newly formed CE from HDL to an apoB containing lipoprotein pool (Lagrost, 1997).
All lipoprotein classes exhibit a net negative charge, due to both the apolipoprotein composition and its content of bound charged lipids (Davidson et al., 1994). However, individual bound lipids forming part of a lipoprotein can contribute either a net positive or a negative charge, or no charge at all, to the lipoprotein. Some phospholipids, when unbound, are negatively charged, some are positively charged, and some are electrically neutral. Examples of negatively charged phospholipids are phosphatidylinositol, phosphatidylserine, phosphatidylglycerol and phosphatidic acid. An example of an electrically neutral phospholipid is phosphatidylcholine. An example of a positively charged (cationic) phospholipid is dioleoyl trimethylammonium propane.
Williams U.S. Pat. No. 6,079,416 teaches administration of large liposomes containing phospholipids substantially free of sterols to treat hypercholesterolemia. Parker et al U.S. Pat. No. 5,614,507 teaches the injection of a bolus of phospholipid, with or without another electrically neutral lipid, to treat endotoxemia. However, the phospholipid compositions disclosed by Williams and Parker et al. are not electrically charged, so they would not act to change the electric charge of lipoproteins within the bloodstream. Instead, they act as a simple adsorbant to pick up and clear cholesterol or endotoxin from the blood.
U.S. Pat. No. 5,652,339 (Learch et al) and U.S. Pat. No. 5,128,318 (Levine et al) disclose the preparation of reconstituted high density lipoprotein (rHDL) particles, and suggest that rHDL particles may be used for drug administration and for treating diseases connected to lipids and lipodal substances. These rHDL particles said to be useable both in vivo or in vitro for removing lipid soluble materials (e.g. cholesterol, endotoxins) from cells or body fluids and aid in the treatment of hyperlipidemia and coronary atherosclerosis. Although, all lipoprotein classes exhibit a net negative charge as discussed above, these two patents teach nothing to increase the charge from that present in normal HDL.
Phosphatidylinositol (PI) is a negatively charged phospholipid found in all classes of lipoproteins and accounts for approximately 4% of the total phospholipid (PL) in HDL (Davidson et al., 1994). Incubation of PI with plasma or with isolated HDL, LDL or VLDL in vitro has shown that all of these lipoproteins can spontaneously absorb PI. However, little is known of what affects, or regulates, the amount of PI in different lipoprotein classes.
There is a need for novel compositions capable of enhancing hepatic clearance of lipoprotein particles thereby lowering cholesterol and tissue cholesterol, endotoxins and other lipid-soluble compounds such as some viruses and bacteria that associate with lipoprotein particles in vivo. There is also a need for methods to make use of such compositions. In particular, there is a need for such compositions, and methods for their use, which will preferentially lower the cholesterol associated with LDL. There is also a need for compositions, and methods for their use, which will retain drugs which associate with lipoproteins in the bloodstream, to increase the duration of the efficacy of such drugs.